The present invention relates generally to optical scanning devices, and more specifically to an apparatus for providing spot position control of a laser spot on a photoreceptive element in a raster output scanning system.
Although applicable to a wide variety of optical output devices, the present invention finds particular utility in Raster Output Scanning (ROS) apparatus. Therefore, the following details and descriptions begin with a background of the present invention in terms of ROS apparatus. ROS has become the predominant method for imparting modulated light information onto the photoreceptor in printing apparatus used, for example, in digital printing, and has found some application in other image forming operations such as writing to a display, to photographic film, etc. Consider, for illustration purposes, what is perhaps the most common application of ROS, digital printing. As is known, the scanning aspect thereof is conventionally carried out by a moving reflective surface, which is typically a multifaceted polygon with one or more facets being mirrors. The polygon is rotated about an axis while an intensity-modulated light beam, typically laser light, is brought to bear on the rotating polygon at a predetermined angle. The light beam is reflected by a facet and thereafter focussed to a "spot" on a photosensitive recording medium. The rotation of the polygon causes the spot to scan linearly across the photosensitive medium in a fast scan (i.e., line scan) direction. Meanwhile, the photosensitive medium is advanced relatively more slowly than the rate of the fast scan in a slow scan direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. (Although this discussion is in terms of ROS apparatus, by way of example, there exist many other scanning and non-scanning system embodiments of the present invention. As a convention, however, the word "scan" will be used when referring to fast and slow scan directions with the understanding that actual scanning of the spot is not absolutely required.) The light beam is intensity-modulated in accordance with a serial data stream at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as sheet paper.
Data in each of the fast and slow scan directions is generally sampled. The sampling rate of the slow scan direction data equates to 300 lines per inch or more in many printing apparatus. It has been shown that errors in the slow scan direction of greater than 10% of the nominal line spacing may be perceived in a half tone or continuous tone image. This implies a need for a high degree of spot position control in the slow scan direction on the image plane, especially in such applications as multiple beam and multiple ROS color printers where control of the position of multiple spots is critical. Furthermore, high resolution printing, on the order of 600 spots per inch or higher demands very accurate spot positioning.
Errors in the spot position in the slow scan direction arise from many sources, including polygon and/or photosensitive medium motion flaws, facet and/or image plane (e.g., photosensitive medium) surface defects, etc. These errors are most commonly addressed by passive or active in-line optics. Positional errors which extend over an entire scan line are most commonly compensated for by retarding or advancing the start of scan or image plane position for that line (this correction being limited to whole multiples of a scan line spacing, thus requiring only passive in-line optical elements). See, for example, Advances in Laser and E-O Printing Technology, Sprague et al., Laser Focus/Electro-Optics, pp. 101-109, October 1983. Another approach employing passive optics is the use of extremely high quality optical and mechanical elements. This necessarily implies higher overall costs, and possible limitations on the durability of the system. Still another example of passive optical correction is the system disclosed in U.S. Pat. No. 4,040,096, issued Aug. 2, 1977 to Starkweather which accommodates a basic polygon ROS structure having runout and/or facet errors (both scanning errors in the slow scan direction) by locating a first cylindrical lens in the pre-polygon optical path, which focuses the beam in the slow scan direction onto the facet, and a second cylindrical lens in the post-polygon path, which focuses the facet onto the desired image plane. Toroidal elements and concave mirrors have also been used to accomplish the same function.
Active compensation for process scan direction errors usually involves a closed loop and/or memory-fed compensation system. A closed loop acousto-optical (A-O) compensation system is discussed in Laser Scanning for Electronic Printing, Urbach et al., Proceedings of the IEEE, vol. 70, No. 6, June 1982, page 612, and the reference cited therein. As discussed in this reference, a slow scan spot position detector is placed in the scan line which, together with related processing apparatus, is capable of quantifying the slow scan displacement. An A-O element is disposed in the optical path whose refractive index may be varied by establishing therein an acoustic wave. A variation in the acoustic wave generated in the A-O element is accompanied by a variation in the dispersion angle (that is, the angle of the output beam relative to the angle of the input beam). The slow scan displacement information from the detector and processing apparatus is fed to the acoustic wave generating portion of the A-O device, which may then control the slow scan direction position of the scan line in response to the displacement information. Further, the control information for certain recurrent displacement errors may be measured in advance and synchronized with the angular motion of the rotating polygon, as discussed in the above reference. See also Visibility and Correction of Periodic Interference Structures In Line-by-Line Recorded Images, J. Appl. Phot. Eng., vol. 2, pp. 86-92, Spring 1976.
Copending United States patent applications Ser. Nos. 07/747,166 and 07/747,889 disclose an apparatus and method of slow scan direction spot position control in which spot position control is achieved by interposing between the light source and photoreceptor an optical element which deflects incident light in the slow scan direction by an amount which depends on the wavelength of the incident light. Copending United States patent applications Ser. Nos. 07/747,039 and 07/747,176 disclose an apparatus and method of slow scan direction spot position control in which spot position control is achieved by interposing between the light source and photoreceptor an electro-optical element which deflects incident light in the slow scan direction by an amount which depends on the electrical voltage applied across it. In both cases, spot position control may be part of a feed back loop, or may be based on predetermined spot position requirements.
There is presently a need in the art for spot position control apparatus and methods which provide improved simplified, very high resolution deflection of an optical beam in the slow scan direction.
Shortcomings of spot position control schemes known in the art include the complexity, cost and/or the difficulty of manufacture of such systems. For example, the use of high quality optics requires not only high quality optical elements, but utmost control in the positioning of those optics, in order to obtain the requisite very precise mechanical control sufficient to adjust spot position 0.02 mm or less, required in many cases. In order to achieve this level of spot position control with the aforementioned acousto-optic modulators, an acoustic wave must be established and maintained with great precision. The acousto-optic modulators employed are relatively quite expensive, and require an associated accurate high frequency signal generator and related electronics to produce and maintain the acoustic waves.
Two further disadvantages of many prior art spot position control schemes are the speed and precision at which they are capable of operating. For example, three of the most common ROS schemes, cylinder lenses, rotating mirrors, and translating roof mirrors are generally too slow to correct for motion quality errors or line-to-line errors, while rotating mirrors and translating roof mirrors are also large and therefore difficult to move precisely and quickly.